The formation of sulfuric and nitric acids, which are the primary components of acidic deposition, involves many homogeneous gas phase and aqueous phase reactions, as well as heterogeneous reactions on the surfaces of solid particles (see e.g., Finlayson-Pitts and Pitts, 1986, Ch. 11). In power-plant plumes, the observed rates of oxidation of SO2 are typically less than 10 percent per hour. But much higher rates are often observed when the plume passes through a cloud or fog bank. Rates of conversion appear to be higher at noon than at nighttime and generally higher in summer compared with the winter season. These diurnal and seasonal effects indicate the importance of photochemistry and, perhaps, temperature, in the oxidation of SO2 (Finlayson-Pitts and Pitts, 1986). The rates of oxidation of SO2, both in plumes and in ambient air, are generally higher when hydrocarbons, nitrogen oxides, and the host of secondary pollutants they pro-duce are also present. The presence of liquid water in aerosols, fog, and clouds is also an important factor in determining the overall rate of conversion of SO2 into sulfates including H2SO4. The possible reactions involved in the gas phase, liquid phase, and on aerosol surfaces, which may contribute to the observed rates of conversion of SO2, are reviewed elsewhere (Finlayson-Pitts and Pitts, 1986; Seinfeld, 1986).

The oxidation of NOx in power-plant plumes and in ambient air into nitrates and nitric acid has not received as much attention with regard to acidic deposition, because SO2 is considered to play a dominant role in most regions of concern in Europe and the northeast United States. However, in power-plant plumes, rates of conversion of NOx between 0.2 and 12 percent per hour have been observed. These have similar diurnal and seasonal variations as SO2 conversion rates. Conversion rates in the so-called urban plumes, that is, the ambient air that has passed over urban areas, are also high (up to 24% per hour). Most of the NOx is converted to HNO3 and PAN; the various homogeneous gas phase and aqueous phase reactions involved in conversion are reviewed by Finlayson-Pitts and Pitts (1986). Which phase is most important depends on the particular meteorological conditions, such as temperature, relative humidity, intensity of solar radiation, and presence of clouds.

Chemical submodels used in long-range transport and regional acid deposition model range from overly simplistic models using only one lumped reaction representing the conversion of SO2 into H2SO4 with an arbitrarily specified conversion rate constant to highly sophisticated carbon bond mechanism and other chemical kinetic mechanisms (see e.g., Zwerver and van Ham, 1985; Finlayson-Pitts and Pitts, 1986; Chang et al., 1987). Rodhe et al. (1981) first incorporated the simplified reactive hydrocarbon and NOx chemistry in their long-range transport model for sulfuric acid formation and deposition. Since then, a number of long-range transport and regional acidic deposition models using sophisticated chemistry submodels have been proposed (Seigneur and Saxena, 1984; Venkatram et al., 1984; Chang et al., 1987). For example, the chemistry submodel of Seigneur and Saxena (1984) includes eighty gas phase reactions, twenty-seven liquid phase reactions, and twelve equilibra describing the gas-liquid partitioning.

With such a sophisticated chemistry module for the transformation of complex pollutants, the most recently developed urban air quality models always are required to incorporate realistic descriptions of emissions, meteorology, chemistry and removal processed to obtain the temporal and special resolution. Thus the three-dimensional Eulerian grid models are then becoming more and more important.